Phonon generation handling and detection

ABSTRACT

Microwave pumping of an alkali halide crystal doped with hydroxyl electric dipole impurity ions and subjected to a (001) dc electric field, will increase the population of the 3A1 dipolar energy level (the energy level notation applies to the usual case of preferred alignment of the hydroxyl dipoles along the (100) axes, such as in KC1) of the impurity ion when the microwave frequency is in resonance with the 1A1 -&gt; 3A1 transition. Relaxation rates between various dipolar energy levels are shown to produce a population inversion between the 3A1 and 1B1 states and also between 2A1 and 1A1 states. Either spontaneous relaxation or stimulated relaxation from 3A1 -&gt; 1B1 produces phonons having a preferred transverse polarization and a direction of propagation along the (110) axes of the crystal system. Tuning of the output beam of phonons over a phonon frequency range of about 109 to 1011 Hz is achieved by a variation of the dc field. Variation of phonon absorption of an unpumped crystal in accordance with applied dc electric field and the depolarization induced by such absorption are employed to achieve modulation and detection, respectively, of a transmitted phonon beam.

United States Patent Silvera et al.

[151 3,693,104 [451 Sept. 19, 1972 [54] PHONON GENERATION HANDLING ANDDETECTION [72] Inventors: Isaac F, Silvera, Thousand Oaks; Lawrence A.Vredevoe, Santa Monica, both of Calif.

[73] Assignee: North American Rockwell Corporation [22] Filed: June 5,1970 [2]] Appl. No.: 43,731

[52] US. Cl ..330/5.5, 181/05 R, 329/1, 330/45, 331/94, 332/31 [51] Int.Cl ..H0ls 1/00 [58] Field of Search ..330/5.5; 181/05 R, 0.5 AG, 181/05J Primary Examiner-Roy Lake Assistant Examiner-Darwin R. HostetterAttorney-L. Lee Humphries, H. Fredrick Hamann, Thomas S. MacDonald andAllan Rothenberg [57] ABSTRACT Microwave pumping of an alkali halidecrystal doped with hydroxyl electric dipole impurity ions and subjectedto a (001) dc electric field, will increase the population of the 3A,dipolar energy level (the energy level notation applies to the usualcase of preferred alignment of the hydroxyl dipoles along the (100)axes, such as in KCl) of the impurity ion when the microwave frequencyis in resonance with the 1A 3A transition. Relaxation rates betweenvarious dipolar energy levels are shown to produce a populationinversion between the 3A and 1B, states and also between 2A and 1A,states. Either spontaneous relaxation or stimulated relaxation from 3A1B produces phonons having a preferred transverse polarization and adirection of propagation along the (l 10) axes of the crystal system.Tuning of the output beam of phonons over a phonon frequency range ofabout 10 to 10 Hz is achieved by a variation of the dc field. Variationof phonon absorption of an unpumped crystal in accordance with applieddc electric field and the depolarization induced by such absorption areemployed to achieve modulation and detection, respectively, of atransmitted phonon beam.

14 Claims, 7 Drawing Figures 19 m2 3 I 693. l 04 SHEET 1 0F 2 ZERO FIELD2/000 (on) f A \l u Li 0 X000) {/10} Halo g E -2A /A/ "E0 u APPLIEDFIELD (00/) FIG. 2 m0} Y(0/0) FIG. 3

INVENTOR$ ISAAC F. SILVE'RA LAWRENCE A. VPEDEVOE BYi 97 ATTORNEY PMEIEBSEP 1 9 I972 SHEET 2 0F 2 FIG. 4

/4 m8 2 H mm NM 8 WM TUNABLE MICROWAVE FIG. 5

PHONON DETECTOR .?2 W [TIT A W PHONON SCOURCE INVENTORS. ISAAC F SILVERALAWRENCE A. VEEDEVOE A TTORNE Y PHONON GENERATION HANDLING AND DETECTIONBACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The presentinvention relates to high frequency sonic energy beams and moreparticularly concerns apparatus and methods for generation,amplification, modulation and detection of phonons.

2. DESCRIPTION OF PRIOR ART Sonic energy has long been employed in avariety of applications, scientific studies, and many types of practicaldevices. In various arrangements, sonic energy transducers have beenemployed for measurement and inspection of materials and structures. Abeam of sonic energy is transmitted and reflected in different amountsin accordance with gross discontinuities such as, for example, voids inthe bonding of a laminated structure. This provides informationconcerning internal structural aspects that is not readily available byother observation or inspection techniques. Microwave sound waves arebeginning to find application in sonic circuits that are analogous inmany respects to electrical and fluid circuits. Microwave sonic beamsare useful in the study of molecular and atomic structures of materials,their characteristics, and their physics. For example, phononspectroscopy provides valuable information concerning impurities in asolid body and the relation of such impurities to the host material.

The study of material properties by means of ultrasonic or hypersonic(microwave) sound waves has several advantages over study by othermeans, such as optical techniques. Because the interaction mechanismsare different, materials that are opaque to electromagnetic waves can bequite transparent to sound waves, enabling one to nondestructivelyexamine the interior of such materials. Furthermore, materials that areoptically transparent may have properties such as strains and defectsthat cannot be observed optically, but can be easily differentiated byhigh frequency sound waves.

It is of advantage to work with higher frequency sound waves becausespatial resolution is limited by the wavelength of the vibration. A 10GHz microwave with a typical sound velocity of 10 cm/sec has awavelength limit to the resolution of about cm. At room temperature theuse of hypersonic waves is limited to thin samples due to intrinsicattenuation and losses in most materials. These losses are greatlyreduced at low temperatures.

Obvious limitations of lower frequency sound sources have resulted inincreased interest in devices for generation of microwave phonons in thefrequency range of 10 to 10 Hz. Quanta of sonic or vibrational waveenergy, known as phonons, may be generated by the lattice displacementsof a crystal or vibrational motion of ionic impurities of the crystal,or by interaction between these. Phonon generators, and in particular,phonon masers have been studied in paramagnetic systems. For example, C.Kittel, in Phonon Masers and the Phonon Bottleneck," Physical ReviewLetters, Volume 6, No. 9, dated May 1, l96l, suggests feasibility of useof paramagnetic ions to produce maser action. R. Orbach, in The PhononMaser, Physics Letters, Volume 15, No. 1, dated Mar. 1, 1965, proposespopulation inversion by optical pumping of paramagnetic ruby underspecified conditions for phonon maser action.

Observations of ultrasonic maser action in paramagnetic systems aredescribed in E. B. Tucker, in "Amplification of 9.3-kMc/sec UltrasonicPulses by Maser Action in Ruby, Physical Review Letters, Volume 6, No.10, dated May 15, 1961, and P. D. Peterson and E. H. Jacobsen, in MaserAmplification of 9.5-Gigahertz Elastic Waves in Sapphire Doped withDivalent Nickel Impurity Ions, Science, Volume 164, dated May 30, 1969.However, as compared with paraelectric systems, the paramagnetic systemshave a relatively weak coupling of the paramagnetic ion to the lattice.In practice and theoretically, such devices provide less phonon outputpower. In the paramagnetic system, 7

more of the released energy is electromagnetic than in paraelectricsystems.

A paraelectric system, one involving electric dipoles rather thanmagnetic dipoles as an impurity, has an intrinsic, strong dipolecoupling to the lattice which is many orders of magnitude stronger thanthe coupling of magnetic ions. Further, the relatively long spin-latticerelaxation times of paramagnetic impurities prevents the generation ofballistic pulses of phonons. The low saturation power levels and weakmicrowave absorption as compared with paraelectric system, reducesmaximum power available. For reasons such as these, paraelectric phonongeneration is of increased interest. Paraelectric Heating and Coolingwith OH- Dipoles in Alkali Halides" by U. Kuhn and F. Luty, Solid stateCommunications, Vol. 4, pp. 31-33, 1965, suggests a paraelectric phononmaser that population inversion of certain paraelectric energy levels bya rapid reversal of applied electric field. However, the devicesuggested by Kuhn and Luty depends upon reversing polarity of a largeelectric field, in the order of kilovolts, within a time interval thatis small compared with the relaxation times of phonons of the energylevels of interest. Such polarity reversals of large electric fieldswithin times less than about 10 seconds are not readily achievable inpractical applications.

Paraelectric Resonance of OHDipoles in KCl, by G. Feher, I. W. Shepherd,and Herbert B. Shore, Physical Review Letters, Volume 1A,, 2A,, datedMar. 21, 1966, discusses paraelectric resonance of hydroxyl dipoles in apotassium chloride crystal, and speculates that saturation of an energylevel transition will cause a population inversion analogous to thethree level maser. However, the paper by G. Feher, et al., does not showor suggest particular levels of population inversion or power levels,and provides no indication that a useful gain is possible. Withoutidentification of particular levels of population inversion, outputfrequency of course cannot be identified. A practical operating devicecannot be built without a knowledge of the various parameters that arerequired to achieve operation and use.

The need for a useful phonon detector is recognized by C. H. Andersonand E. S. Sabisky in their article Sensitive Tunable Acoustical PhononDetector, Physical Review Letters, Volume 18, No. 7, dated Feb. 13,1967. Anderson and Sabisky suggest a narrow band phonon detector in theGHZ range and employ optical monitoring of spin temperature. Such adevice is limited by its required application of an optical beam and intemperature dependence.

Accordingly, it is an object of the present invention to provide amethod and apparatus for generation, amplification, modulation, anddetection of phonons in a paraelectric system.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows arrangement andidentification of axes used to describe the system,

FIG. 2 illustrates zero and applied field electric dipole energy levelsin a paraelectric crystal,

FIG. 3 illustrates a geometrical relation between crystal axes, appliedfield and polarization directions in a phonon generator, and maserconstructed according to the principles of the present invention,

FIG. 4 illustrates a phonon detector,

' FIG. 5 shows an arrangement for application of the phonon generatorand detector of the present invention to observation of a samplematerial,

FIG. 6 shows an alternate arrangement of phonon generator and phonondetector, and

FIG. 7 illustrates a phonon power modulator.

GENERAL THEORY The present invention, as will be readily understood fromthe following description, is applicable to a wide variety of crystalsdoped with an electric dipole impurity. Nevertheless, for purposes ofexposition and in order to better afford a full and completeunderstanding of the principles of the invention, it will be describedin connection with a potassium chloridehydroxyl system. In such asystem, the QI-Iions substitute for the chlorine ions in the crystallattice and there is, in effect, a KClzKOH system.

Tunnelling and librational types of motion of dipolar impurities such asthe hydroxyl ion within the alkali halide lattice are controlled by amultiwell crystal field potential that exists at the substitutionallattice site, the side of the impurity ion. For an infinitely highpotential maximum, each quantum state of dipole orientation isdegenerate in accordance with periodicity of the crystal potential andmay be represented by a localized state in a specific direction. Forpotential barriers of finite value, these localized states overlaythereby afford a small amount of energy level splitting. This is asolid-state tunneling. For the hydroxyl ion in the potassium chloridecrystal energy level splittings are in the microwave frequency range forthe lowest levels of tunnelling states. The hydroxyl ion impuritypossesses energy minimal for orientations of its electric dipole momentalong the fixed 100) axes of the KC] host.

Briefly digressing, it is noted that various axes of the crystal areconventionally identified according to notation shown in FIG. 1 where x,y, and z identify three mutually orthogonal axes and the axis (001)corresponds to z, the axis (010) corresponds to y, and the axis (100)corresponds to x. To identify axes intermediate to these, the variousnumber symbols are combined so that for example axis (101) identifies anaxis lying in the ZX or (001) (100) plane and bisecting the x and zaxes. Similarly, the symbol (110) represents an axis lying in the xyplane and bisecting these axes.

To continue, in the absence of an applied dc field, the tunneling statesmay be expressed as illustrated in FIG. 2 as E a doublet state of evenparity, T", a first triplet state of odd parity and A a first singletstate of even parity. FIG. 2 illustrates these energy levels as measuredalong the ordinate in the graph as they vary with application of anapplied dc field measured along the abscissa. The field is applied alongthe (001) axis of an hydroxyl ion doped potassium chloride crystal. Asmay be seen in FIG. 2, the zero field dipolar energy levels are furthersplit by the application of an external dc electric field, there beingdifferent splittings depending upon the axes along which the field isapplied. Quantitative values for the tunnelling splitting A of the fieldstates of the hydroxyl ion tunnelling in a potassium chloride lattice,and for the effective dipole moment P, of the system can be obtainedfrom analysis of the paraelectric resonance of various microwavefrequencies as described, for example, in Tunnelling States of OH in KClCrystals by W. E. Bron and R. W. Dreyfus, Physical Review, Volume 163,No. 2, pp. 304-314, Nov. 10, 1967. Briefly describing such measurement,a specified value of dc field is applied, the doped crystal is subjectedto a microwave electric field in a microwave cavity, and both themicrowave frequency and dc field magnitude are varied. For any givenapplied dc field, there is a maximized absorption of microwave energy ata given microwave frequency, due to electric dipole transition betweenthe various energy level states. Thus quantitative values of the energylevels and their variation with applied field may be identified.

As is well known, the various energy levels in a multilevel systemgenerally tend to exhibit decreased numbers of particles, in this caseionic dipoles, in levels of increasing energy. Thus, for a system suchas that illustrated in 'FIG. 2, the lowest level 1A,, the ground state,would tend normally to be most heavily populated. Stated in anotherfashion, the number of dipoles in the ground state, 1A,, is normallygreater than the number of dipoles in higher energy levels. Further,dipoles in higher energy levels tend to relax or decay into lower energylevels. Relaxation rates for the transitions of dipoles from higher tolower energy levels vary widely in different systems and in differentmaterials. The relaxation rate may be considered to be the average timethat it takes for a dipole to go from a higher level to a lower level.In such a transition, the difference in energy is given up as a phononor a photon.

A significant advantage of use of paraelectric systems for phonongeneration resides in the fact that the energy level transitions in sucha system yield considerably greater portions of released energy in theform of phonons, whereas in paramagnetic systems, more of the energyreleased in such a transition is in the form of photons.

In general, there is a tendency for more widely separated energy levelsto undergo spontaneous transitions or relaxation more quickly thanlevels that are closer together in energy. This is supported bycalculations of dipole lattice relaxation rates that are in agreementwith experiment. Such calculations are based upon assumption of a modelin which the center of charge of the hydroxyl ion is displaced from thechlorine ion lattice side by about 0.3 angstroms in the direction of thedipole moment. Such calculations are set forth in Relaxation ofOHDipoles in KC] at Low Temperatures, by Lawrence A. Vredevoe, ThePhysical Review, Volume 153, No. 2 pp. 312-318, Jan. 10, 1967, and showthat for the case of (001) do electric field, the dipole latticerelaxation rates for lower energy transitions, from level 2A 1A and fromlevel 3A 1B,, are significantly slower than those for higher energytransitions 3A 2A, and 113, 1A,.

With this model, dipole lattice relaxation times have been calculatedusing measured values of the hydroxyl dipole moment P-3.3 X esu and zerofield splitting values of A-5.5 X 10 Hz. In the presence of a dc field,E, 10 KV/cm parallel to the (001) axis of a potassium chloride hostcrystal, the calculated dipole lattice relaxation times for atemperature much smaller than the product of applied field and dipolemoment are 1.1 X 10' and 1.3 X l0" seconds for lower energy transitionsfrom levels 2A, LA, and from level 3A, to level 18,, respectively. Theserelaxation times are 5.6 X 10 and 4.2 X 10" seconds for the higherenergy transitions, from level 3A to level 2A and from level 18 to level1A respectively. Since relaxation times for such transitions by means ofphoton emission are in the order of 10' seconds, electromagnetic orphoton contribution to these relaxation rates is negligible. Withrelaxation times as calculated above, the ratio of the dipole latticerelaxation rates 3A, 2A /2A 1A,, and the ratio of relaxation rates 18,lA /3A 18, are 2 and 3.2, respectively.

Accordingly, it will be seen that if level 3A is provided with a highpopulation density whereby the various transitions identified above mayoccur at the identified decay rates, the ratios 2 and 3.2 wouldi'esultin two population inversions. A first inversion occurs between 3A andllB states and a second between 2A and 1A, states. This is analogous tothe three level maser described in Proposal for a New Type Solid StateMaser" by N. Bloembergen, Physical Review, Volume 104, No. 2, Oct. 15,1956. For example, with maximized population density in level 3A,, thedecay rate to level 113 is relatively slow, whereas decay rate fromlevel 1B, to ground state level 1A is relatively fast. Thus, level 13,becomes depopulated more rapidly, whereas level 3A retains a higherpopulation, resulting in a population inversion wherein the populationof 3A is greater than that of level 13,. Similarly, since the decay ratefrom 2A, 1A, is less than the decay rate from 3A, 2A,, there results anincrease in population of level 2A, with respect to population of 1A,,to achieve a second population inversion.

In the presence of. relatively large dc electric fields, in the order ofor greater than 6 KV7cm parallel to the (001) axis, energy (and,therefore, frequency) of phonons produced in transition between the twopairs of inverted levels are equal (although power is greater for the 3A1B, transition). This transition energy is proportional to the appliedfield E,,. In other words, as the applied field E is increased, theenergies of the transitions will increase. Thus for dc electric field inthe range of about 6 to 50 KV/cm, the emitted phonon energy can bevaried (tuned) from about 10 to 10 Hz.

Energy level transitions in the presence of a (001) do electric field,reveal another significant aspect of these non-radiative (phonon)transitions. As described in the paper by Vredevoe identified above,these relaxation processes are dominated by phonons with lowestvelocity. There is also an angular dependence in the relaxationmechanism that favors certain axes. Ac-

cordingly, it is found that the 3A, 2A, and the 2A 1A transitions couplemost strongly to transverse phonons that are propagating parallel to the(101) or (011) axes. The SA, 1B, and the 113 1A transitions are found tocouple most strongly to transverse phonons propagating parallel to theaxis. Transverse phonons are vibrations in shear, occurringperpendicular to the direction of propagation of the acoustic wave.Except forthe effects of anharmonicity and other decay mechanisms, thedipolar energy that is lost in the above-identified transitions isselectively pumped into transverse phonons that are propagated parallelto the (110) axis of the cubic crystal.

Thus, it will be seen that given a suitably dense population of theupper level 3A,, a pair of population inversions will occur, resultingin transitions that yield energy into transversely propagated phonons.

It has been shown in Microwave Saturation of Paraelectric-ResonanceTransitions of OHIons in KCl, by L. D. Schearer and T. L. Estle, SolidState Communications, Volume 4, pp. 639-642, 1966, that the IA, 3Atransition can be saturated at relatively low microwave powers in thepresence of a 3KV/cm dc field at a temperature of l.3 K. For larger dcfields, the pumping power required to saturate this transition islarger. For example, microwave powers on the order of 1 watt are neededto be supplied to a TB, cavity with a loaded Q of 4,000 in order tosaturate the 3A level in a dc field of IO KV/cm at 1 K.

Accordingly, combining the fact that significant microwave pumping tothe 3A, level is possible, with the ratios of decay rates describedabove, and solving the rate equations for the system, enables design andconstruction of apparatus for tunable and directional phononamplification via stimulated emission of transverse phonons along the(110) axis of the paraelectric system.

Amplification of propagated phonons can be enhanced along a given (110)axis by preparing the corresponding surfaces (normal to this axis) ofthe excited doped crystal flat and parallel, to enhance elasticreflection of the (110) axis phonons in a Fabry- Perot geometry. As wellknown, Fabry-Perot geometry employs multiple reflections between facinginternal surfaces to afford enhancement of the propagated signal. Thegenerated phonons belonging to the lowest or transverse branch of thephonon spectrum also possess anomalously long lifetimes insofar asanharmonic decay is concerned, thus still further enhancing buildup ofphonon population in these particular modes of transverse propagation.

A particular example of required pumping rate to achieve populationinversion between 3A, and 18 levels of an exemplary system may becalculated as described below.

Solution of the equations for relaxation rates of the system fortemperatures at which kT is small compared with the filed splitting, A,of the dipolar levels shows that the steady-state pumping power requiredto produce population inversion between the 3A, and IE levels is lessthan that needed to produce inversion between the 2A and 1A levels. (kis Boltzmanns constant and T is temperature in degrees Kelvin.) Forpopulation inversion between these levels it is required that thepopulation ratio,

be greater than unity in the presence of large dc fields (where E, isgreater than about 6 KV/cm). In order to obtain such an inversion, it iscalculated that the pumping rate must be as given in equation l Thepertinent equations are set forth at the end of this specification. Forthis calculation, it is assumed that the pumping rate is sufficientlylarge relative to the dipole lattice relaxation rates, that the phononscreated in the active modes will not appreciably alter effectivetemperature of these modes from that of the lattice during the period ittakes to achieve the inverted population. Pumping rates required byequation (1) increase strongly with temperature, thus making itadvantageous to work at as low a temperature as possible.

PHONON GENERATOR A specific example is considered wherein a 10 KV/cm dcelectric field is applied parallel to the (001) axis of a potassiumchloride crystal doped with an hydroxyl ion. The crystal is located inthe electric field of a TE microwave cavity. Confocal or re-entrantmicrowave cavities may also be employed to provide the benefits ofhigher Q and increased electric field for a given power input, andlarger sample area. The resonance frequency for the pumping transitionfrom level 1A to level 3A in this example is 35 61-12. For a latticetemperature of 1 K, a microwave pumping power of 500 milliwatts suppliedto the cavity produces a 12 percent inversion between the 3A and 18states. If the lattice temperature could be maintained at the 0.5 K, apumping power of only 180 milliwatts'would be required to the cavity toproduce this 12 percent population inversion between these states. Thus,advantages of working at lowest possible temperature are apparent. Inany event, crystal lattice temperatures that are significantly abovethese low levels will cause temperature induced vibrations of thelattice that significantly mask any desired single frequency phononpropagation.

The gain for the above-described phonon emission and amplificationsystem indicates that significant phonon powers are generated even witha relatively small amount of electrical dipole impurity in the the hostcrystal. Gain is calculated in a manner analogous to that used forcalculation of the gain of an optical maser. Assuming a Lorentz lineshape for the strain broadened dipolar levels, there is derived anexpression for gain (in the presence of large dc fields) for phononsgenerated by the 3A 1B dipole lattice relaxation transition. Thestrongest contribution to the gain is obtained with the phonon-dipolecoupling terms that arise from displacement (from the centrosymmetricsite) of the center of charge of the dipole in the direction of itspolarization. This non-centrosymmetric contribution to gain is set forthin equation (2).

Although the above-described phonon emission and amplification can beachieved with various types of impurity doped crystals, the latterfrequency exhibit a strong inhomogeneous strain broadening of thedipolar levels. Such strain broadening will result in an increased bandwidth of the desirably narrow, monochromatic phonon beam that isgenerated. Accordingly, to

minimize such bandwidth and to limit the strain broadening of dipolarenergy levels, it is preferred to employ crystals having minimalinternal stress, such as for example, doped crystals that are annealedas described in Paraelectric Resonance of Annealed KClzKOH Crystals byR. W. Dreyfus, Solid State Communications, Volume 7, pp. 827-829, 1969.

For such annealed crystals, and using a half-width of observed dipolartransition of 8v-6 X 10 Hz, equation (2) indicates a gain of about a 7.4cm. Such a gain is yielded by an hydroxyl ion concentration of KOH/KCI10", or one part per million, and a dc field E, IOKV/cm. Frequency ofthe 3A 18, transition is 12 GHz. This gain is achieved at 'a crystaltemperature of 1 K and'employing 500 milliwatts of microwave of pumppower. For temperature of 0.5 K and 180 milliwatts of pump power, thegain would be 4.5 cm".

It should be observed that the above-identified gains are for anhydroxyl ion impurity concentration of the relatively low level of onepart per million. The described mechanism of phonon generation isdependent upon the total number of hydroxyl ions involved, wherebyincreased concentrations will increase the phonon power that isproduced. Accordingly, impurity concentrations of up to 10 or 20 partsper million will provide 10 to 20 fold increase in the identified gains.Concentrations of hydroxyl ion of up to parts per million are possiblewith alkali halide crystals. However, for concentrations beyond about 10parts per million the mechanism of phonon generation begins to bemodified somewhat. At greater impurity concentration, the individualdipole ions no longer are sufficiently separated to allow their actionsto be considered relatively independent of each other. For such greaterconcentrations, the phonon emission mechanism is changed by the actualinteraction between adjacent impurity ions.

Illustrated in FIG. 3 is a schematic arrangement of a phonon emitterconstructed in accordance with principles described above. A tunedmicrowave cavity 10 has suitably mounted therein for exposure to theelectric field of the cavity, a paraelectric crystal 12 of the typepreviously described. Cavity 10 is excited via a microwave source suchas a klystron and wave guide (not shown in this figure) at the frequency(such as 35 Gl-lz) of the transition between levels 1A and 3A of thedipole impurities. A dc field from a variable source 14 is applied tothe crystal 12 in the direction of the Z or (001) axis of the crystal.The entire arrangement (except, the several power supplies) is cooled toa temperature of about l K, as by immersion in a bath (not shown in thisfigure) of liquid helium, for example. Thus, when the illustratedcrystal is pumped with microwave frequency and subjected to a dc fieldalong the (001) axis, there occurs an emission of transverse phonons atthe frequency corresponding to the 3A 1B transition, and propagatedalong the (l 10) axis.

A suitable vibrational energy coupling device such as for example asolid sapphire rod or bar 16 has an end thereof mounted in closeabutment with the end of crystal 12 for coupling the phonon beam fromthe generating crystal. The generating crystal 12 extends from andbeyond the microwave cavity. The coupling bar 16 extends from itsabutment with crystal 12 from and beyond the cooling bath, so as toallow coupling of the phonon beam to an output device that is at somehigher temperature. Further details of arrangement and application ofthis phonon generator are described below in connection with FIGS. and6.

When used as a phonon generator, phonons may be released in transitionsbetween the identified inverted population levels by a spontaneousreaction. In addition, for a maser type of action, the relaxation ortransition between inverted population levels is employed to amplify arelatively weak existing microwave sonic vibration. Thus, an inputvibration propagating along the (110) axis at the frequencycorresponding to the transition between the inverted population levels3A and 1B,, for example (l2Gl-Iz with an applied dc field of KV/cm), maybe applied to and significantly amplified by the phonon maser describedherein. Such an input vibration that is to be amplified may be appliedvia a suitable sonic energy coupling rod 17, similar to rod 16 (FIG. 3)and fixedly mounted in close abutment with the end of paraelectriccrystal 12 that is remote from coupling rod 16. The upper end (as shownin FIG. 3) of crystal 12 is preferably within the microwave cavity.However, for use with the input energy coupling rod 17, this upper endextends out of the cavity for connection with the coupling rod. Thus, aweak sonic energy'signal is applied at one end of the paraelectricPHONON DETECTION In the presence of a dc field that polarizes the dipoleimpurities of the paraelectrical crystal, absorption of phonons by anunpumped system will achieve a concomitant dielectric depolarization.Such depolarization may be conveniently used for detection of receivedresonant phonons. The strong dipole lattice coupling in the hydroxyldoped potassium chloride crystals results in a large absorptioncoefficient, or negative gain, for phonons resonant with certain energylevels in an unpumped system. For example, in the presence of a strongdc electric field (where the product of the field strength and dipolemoment is much greater than the zero field splitting, A, and where thefield is parallel to the (0M) axis) the 1A ground state of the system isa state in which the hydroxyl dipole moment is oriented parallel to thedc field. In such a system, however, dipoles in 2A, and 13, intermediatestates have the dipole moments thereof oriented perpendicular to theapplied dc field. This may be observed from the wave functions of thephonons as set forth in equations (3), 4) and (5) below. Equation (5 forground state 1A,, indicates that the coefficient of the Z axis vector isunity, whereas the coefficients of the and Y axes vectors areconsiderably less than unity. Likewise, equation (3) for the wavefunction of state 1B indicates a zero Z axis vector and a substantiallyequal distribution along X and Y axes. Equation (4) for state 2Aindicates a substantially greater distribution in X and Y (by virtue ofthe coefficient one-half) than in the Z axis direction which terms havecoefficients considerably less than one-half. These differences indipole moment orientation in the several states shows that as microwavephonons in resonance with the 1A, 2A, or with the IA, 1B transitions arestrongly absorbed, the

polarizability and the net polarization in the presence of the dc fieldare significantly reduced. This reduction in polarizability of theparaelectric crystal is employed as a tunable and amplitude sensitivedetector of microwave phonons.

A measure of the variation of polarizability or of the actual decreasein polarization, in the presence of an applied dc bias field is achievedby making a differential capacitance measurement of the paraelectriccrystal, using the crystal as the dielectric of a capacitor. Asillustrated in FIG. 4, a phonon generator or source of phonons ofundetermined power and whose presence, power and frequency are to beobserved and detected, is arranged to couple its phonon beam intoparaelectric crystal 20 of the type described above, such as forexample, the potassium chloride material doped with an hydroxyl dipoleimpurity. Electrically conductive plates 22, 24 are positioned adjacentto and in contact with opposite sides of the paraelectric crystal toprovide a dc bias field that is generated by a variable dc source 26. Adifferential capacitance detector 28 is series connected with the plates22, 24 and source 26. The differential capacitance and detectorcomprises an ac source 30 and a meter or oscilloscope 32 connected inseries in the illustrated circuit.

When no phonons resonant at the frequency determined by the magnitude ofapplied dc field are received by the crystal 20, substantially all ofthe dipole moments are oriented in the direction of the field. In asystem analogous to that shown in FIG. 3, this direction is along the Zaxis, for example. The differential capacitance detector 28 provides areading of the capacitance between plates 22, 24, having the relativelypolarized dielectric of crystal 20 therebetween. Now consider generationof a beam of phonons resonant at the 1A 2A or 1A 13 transition of thedoped crystal 20 for a particular value of the applied dc field. As suchresonant phonons are absorbed, a continuous stream of dipoles are raisedfrom the ground state 1A to state 2A or 1B,. In either of these higherenergy level states, the dipole moment is normal to the dc field ratherthan aligned therewith as in the ground state. Accordingly, there is anet decrease in polarization of the crystal 20 in the direction of theapplied dc field. Since the polarization of the capacitor dielectric ischanged, the capacitance detected by the differential capacitor 28 isalso changed by an amount that is related to the power of absorbedphonons.

To detect phonons of a different frequency, the magnitude of applied dcfield is varied to a value that provides a transition frequency betweenground state and 2A or 1.8 of the desired magnitude.

Like the phonon emitter, the detector operates only at temperatures thatare exceedingly low compared to the resonance energy, whereby thepopulation factors MA -N and N -N are maximized. Under such lowtemperature conditions, the impurity bulk polarization for hydroxylconcentration of 1 part per million is as set forth in equation (6)below. Equation (6) relates to the case of resonance with the 1A 2Atransition.

The above described arrangement for phonon detection may be appliedconveniently for. detection of phonon amplification by stimulatedemission, by observation of line narrowing, that is, observation of theill decreased bandwith of the maximized or peak absorption that isdetected. For such an arrangement, the phonon generator 18 is arrangedas illustrated in FIG. 3 and the doped crystal 20 may be a singlecontinuous crystal arranged as illustrated in FIG. 3 to emit phonons atone end thereof within a pumped microwave cavity. The same crystal, forsuch arrangement, has the other end thereof extending from the microwavepumped cavity for phonon detection in the manner illustrated in FIG. 4.

APPLICATION Illustrated in FIG. 5 is an arrangement of phonon generatorand phonon detector constructed in accordance with principles of thepresent invention, adapted for observations on a sample material. Atunable microwave generator, such as a tunable klystron 34, has themicrowave energy thereof coupled via a wave guide 36 to provide anelectric field to a tunable microwave cavity 38 in which is mounted aparaelectric phonon generating crystal 40. The microwave cavity andcrystal are suitably cooled, as to temperature of liquid helium, forexample, by a surrounding cooling chamber 42. A variable dc source 44,providing a field in the order of KV/cm, is coupled to electricallyconductive plates affixed to opposite sides of one end of crystal 40within the microwave cavity.

The klystron microwave generator 34 and cavity 38 are both tunable andboth actually tuned to provide pumping power at a frequency determinedby the transition between states 1A and 3A for the crystal 40 in thepresence of a dc field of a given magnitude. The dc field may be variedin order to tune the frequency of the output phonon beam. As can be seenin FIG. 2, variation of the dc field of source 44 will vary the energylevel difference between the population inversion states 3A and IEwhereby the phonons emitted in transition between these levels will varyin frequency in accordance with the variation of the magnitude ofapplied dc field. In a typical example, the paraelectric crystal 40 maybe in the order of several centimeters in length and about one-half byone-fourth cm in transverse dimensions.

A suitable sonic energy coupling device in the form of a rod of sapphire46 has a cross-sectional dimension substantially the same as that of theparaelectric crystal 40 and has an inner end thereof in close abutmentwith an end of the paraelectric crystal 40. Preferably, mating ends ofthe crystal and sapphire rod are carefully ground and polished formaximum surface contact and are suitably joined by one of a number ofwell known adhesive materials. For example, a material known as Barfaq,which is a viscous paste at room temperature, may be coated on themating surfaces and is solid at liquid helium temperatures to therebyfixedly mount the sapphire rod to the paraelectric crystal. Sapphire isa preferred material for the sonic coupling rod because it exhibits lowattenuation and scattering of phonons propagating through thetemperature gradient that is presented between the cooled crystal andhigher temperatures.

The rod 46 may be of circular cross-section or may be of a thin strip,preferably of at least a millimeter in diameter, and having a length ofas much as several inches. Minimum diameter of the coupling rod 46 isnot limited by frequencies, since wavelength of phonons at frequenciesin the order of 10 to 10 Hz are considerably smaller than any feasibleminimum rod diameter. Preferably, the rod diameter should be sufficientto adequately conduct a phonon energy beam with minimum loss and yetprovide a minimal thermal path to the cold crystal.

It is noted that the coupling rod 46 preferably extends out of theenvironmentally cooled chamber 42 from its junction therein with thecrystal 40 for convenient connection to a sample that may be at roomtemperature. Thus, the outer end of energy coupling rod 46 may be placedin close contact with a sample 48 that is to be subjected to themicrowave phonon beam. Phonon vibrations propagated through sample 48are received by a second sonic energy coupling rod 50,

which may be identical to the rod 46. Rod 50 extends into a secondcooled chamber 52 in which is mounted a phonon detector of the typeillustrated in FIG. 4. The detector comprises a second paraelectriccrystal 54 having dc electric field appliedthereto by a variable source56 and including a differential capacitance de tector formed by an acsource 58 and a meter 60.

It will be readily appreciated that a variety of modes of application ofa phonon beam to a material to be observed may be achieved. Thus, thesonic energy of the phonons may be either transmitted through thesample, may be refracted by the sample, or may be reflected from thesample. An arrangement involving the latter situation is illustrated inFIG. 6. A phonon generator comprises a microwave cavity 62, carrying aparaelectric crystal 64 that is excited by a variable dc field providedby a source 66. The cavity is fed with a suitable microwave pumpingenergy by a microwave source and wave guide (not shown). The generatoris mounted within a cooled chamber 68 and includes a sonic energycoupler such as a sapphire coupling rod 70 fused to an end of theparaelectric crystal 64 and extending therefrom externally of thechamber 68.

The free end of coupling rod 70 is adapted to be contacted by thesurface of a sample material 72 that is to be observed by the manner ofits reflection of phonons. Phonons reflected from the sample 72 arecoupled to a second coupling rod 74 in contact with another area of thesample 72. Coupling rod 74 extends into the cooled chamber 68 and hasits inner end fused or otherwise maintained in close contact with theend of a second paraelectric crystal 76 that is arranged according toprinciples of this invention to operate as a phonon detector.

The phonon detector illustrated in FIG. 6 employs a slightly modifiedarrangement for detecting absorbed phonons by means of depolarization ofthe paraelectric crystal. This crystal is also subjected to a dc fieldsupplied by a variable source 78 along its (001) axis, for example.Again, the paraelectric crystal is employed as the dielectric betweenthe plates of a capacitor that is provided with a dc bias by source 78.Upon receipt of phonons transmitted by coupling rod 74, the dipolarstates of impurities of crystal 76 undergo a transition between groundstate and 1B, or 2A states. Thereupon, the change in polarization, P,effects a release of a surface charge ASP, where A is the capacitorsurface area. In effect, there is an electric current thereby providedthrough a resistor 80 that is series connected with the capacitor. Avoltage is generated across the resistor substantially equal to ARSP/t,where R is the resistance of the resistor 80 and t is the lifetime ofthe excited dipolar states. The voltage across resistor 80 may beobserved on an oscilloscope 82 as a function of the electric fieldapplied by source 78. Magnitude of the voltage determines receivedphonon power and magnitude of applied field determines received phononfrequency.

Another manner of using polarization change of the crystal in accordancewith phonon absorption is based upon reaction of the net polarization ofthe paraelectric crystal with an external dc electric fieldperpendicular to the axis of polarization. Such reaction generates atorque between the paraelectric crystal and the applied external field.Depolarization of the paraelectric crystal decreases the value of thetorque to thereby provide a measure of received phonon power.

MODU LATION Since the resonant absorption of phonons involves inducingdipolar energy transitions, variation of the applied dc field to changeor control such transitions may be employed for modulation of phononbeam power. As illustrated in FIG. 7, a source of phonons 84 which maybe substantially identical to that illustrated and described inconnection with FIG. 3, causes propaga' tion of a generated phonon beamalong the (110) axis of a paraelectric crystal 86 of a type previouslydescribed. A variable dc source 88 is arranged to provide a controllabledc field along the (001) axis of the brystal 86. A phonon detector 90 ofthe type illustrated in FIG. or 6 is sonically coupled with an end ofthe doped crystal remote from the source of phonons.

As previously described, the energy splittings of the dipolar states arevaried by a variation of the magnitude of applied dc electric fieldwhereby phonons are resonantly absorbed by the crystal upon applicationof a field from source 88 in accordance with the particular dipolarenergy transition. The absorption coefficient for this process is large.In the particular case of the hydroxyl doped potassium crystal describedherein, for the transition from 1A 2A levels, the coefficient ofabsorption of phonons at resonance is as identified in equation (7)below. For an impurity concentration of .10" at a temperature of l.50 K.and a resonance phonon frequency of 2 X 10 Hz there is a populationdifference N]A1 N2A1= 1.4 X 10 cm', and an absorption coefficient a1,100 cm.

As illustrated in FIG. 7, phonons from source 84 are generated asindicated by arrow 92 and are attenuated in passing through the applieddc field to provide a diminished phonon beam as indicated by arrow 94.If the applied dc field is at the proper value, the 1A 2A transition isin resonance with the frequency of propagated phonons and the phononsare resonantly absorbed. If the applied dc field is at some other value,phonons will pass through the field with intrinsic losses only. As theapplied dc field is varied resonant absorption of the phonon isconcomitantly decreased.

Response of the illustrated modulator is high, its speed beingdetermined only by the pump rate and relaxation times between thedipolar levels indicated. The pump rate depends upon the phonon powersince it is the latter that raises the dipoles from 1A, state to 2A,state. Relaxation time is on the order of IO nanoseconds. As previouslydescribed, operation is most efficient at temperatures of liquid helium.

The use of the hydroxyl ion as the dipole impurity is the identifiedalkali halide is preferred since this impurity ion has the largestintrinsic dipole and accordingly exhibits the strongest latticecoupling. This dipole is more likely to give up energy by phononemission, as distinguished from photon emission. Thus, the nonradiativeenergy emitted is maximized with this type of impurity in a paraelectriccrystal. Nevertheless, it will be readily appreciated that any of theother alkali ha lides may be employed as host crystals.

The strongest paraelectric ions are OH, CN, 0}, and Li. Of these thefirst three can. beeasily doped into any of the monovalent alkali halidecrystals where the paraelectric ions substitute for the anions of thehost. The last ion (Li replaces the cation of these crystals. Thehydroxyl (OI-I) ion has the strongest dipole moment relative to therest, and therefore possesses the strongest coupling to the latticevibrations of the highly polar alkali halide host. Preferred alkalihalides to use with OI-I" (from stability, size compatibility, andpolarity considerations) are KCI, NaCl, KBr, NaBr, LiCl, and LiBr.

The particular energy level notation used herein applies to systemswhere the dipoles have preferred alignment along the axes. Such systemsinclude OH impurities with any of the alkali halides except KI and CNimpurities with any of the alkali halides.

UTILIZATION The microwave phonons produced at low temperatures in theabove-described tunable masing action in paraelectric doped alkalihalide crystals (such as KClzOI-I) have their simplest application inlow temperature phonon spectroscopy. Since many phonon interactions aremost easily studied at low temperatures, it is advantageous to have asource operating in this temperature region. Typical applications thatare useful in this respect are the direct study of the interactionbetween defect states and phonons, the determination of the location ofpoint defects in materials at low temperatures, and low temperaturephonon microscopy. For cases where the phonon coupled effects can beexamined in the alkali halide hosts (e.g., some phonon defect ioninteractions), the phonons can be generated in one region of the sampleand interact in another, without the necessity of tranversing anyboundary between the sample and source. Point defects in the structureof materials can be located by the reflection of microwave phonons,similar to the supersonic reflectoscope used at ultrasonic frequencies.The high frequency microwave acoustic phonons would provide much higherspatial resolution, however.

It has been observed that absorption and reflection of ultraviolet andother electromagnetic radiation by paraelectric ions is significantlyaffected by their orientation. Thus, propagation of a phonon beamthrough a material having a surface formed of the described paraelectriccrystal will depolarize the surface in different amounts in accordancewith the intensity of the phonon beam propagating through the material.The polarization, and thus the rate of phonon incidence, may beinspected by observation of absorption (reflecthe material on a lightbeam is perturbed by vibration.

Thus, a material subjected to a phonon beam would exhibit increasedvibration where coupling of the lattice element is weaker, thusresulting in an increased scattering of transmitted light. Such anarrangement enables visible detection of relatively soft and hard partsof crystals.

The present invention may be employed as previously described for use inphonon spectroscopy and furthermore may find application in phononmicroscopy. The illumination of a subject sample with sound energy willenable observation of features and detail that cannot be detected byeither optical or electron microscopes. For example, light may directlykill certain living organisms, or the required staining process may befatal. Monochromatic, high frequency phonon beam accordingly willprovide high resolution observations not otherwise possible.

In a proposed 10 GHz phonon microscope operating at room temperature oneof the difficulties encountered is the conversion of the phonon outputinto a visual display. As previously mentioned, the phonon excitedparaelectric levels of a paraelectric crystal change the polarization ofthe crystal and the absorption of ultraviolet light. The detector ofthis invention may be used as a visual display transducer operating atlow temperatures. For this application a paraelectric crystal is used asa screen for the diffracted phonons in a phonon microscope. Ultravioletlight is swept across the paraelectric screen in a raster. The detectedultraviolet light signal can then be displayed on a video screen.Transmission will vary across the paraelectric screen according to theintensity of phonons at a given point on the screen. This ultravioletlight when converted to a video-image then provides an optical image ofthe phonon picture.

Applications of microwave phonon generators and microwave phonons. Forexample, a detector may be used for microwave phonon defect detection,visualizing structures in phonon microscopy, microsound circuitdetectors, and the like.

There have been described details of a phonon emitter, amplifier,modulator, and detector employing paraelectric crystals of paraelectricion doped alkali halides and wherein significant gain is achieved withreasonable values of pumping power and applied dc field. Specific valuesof power, frequency, field magnitude and population inversion levels areidentified to exemplify novel structure and arrangement of the describedmethod and apparatus.

The foregoing detailed description is to be clearly understood as givenby way of illustration and example only, the spirit and scope of thisinvention being limited solely by the appended claims.

EQUATIONS [exp ED-11 (rs-l-l) seewhere T is lattice temperature, E, isapplied dc field, P is OH dipole moment and A is zero field energy levelsplitting.

r 2 z] F where e is the electronic charge, P is the hydroxyl dipolemoment, A is the zero filed splitting p is the crystal density, v, isthe velocity of the generated transverse phonons, a is the latticeconstant, r is the displacement of the center of charge of the dipolefrom a centro-symmetric site. N -N,,, is the population (density)differencelbetween the 3A, and 1B, states (which is given by a solutionof the rate equations and depends upon the pumping rate), h is Plancksconstant, and 81/ is the half-width at half-maximum of the 3A 1Bresonance.

is the ratio of the signal induced excitations to the spontaneousrelaxation.

We claim:

1. A phonon emitter comprising an alkali halide crystal having electricdipole impurities,

means for subjecting the crystal to a dc field sufficient to enable saidimpurity to assume energy levels 1A 2A,, 1B,, and 3A,,

said impurity having a rate of decay from level 3A 2A greater than itsrate of decay from level 1A, 1A and a rate of decay from level 113 lA,,greater than its rate of decay from level 3A 18 a pumping means forapplying energy to said crystal at a frequency to raise said impurity tosaictBA level,

7 said pumping means including means tuned to the frequency of the 1A to3A transition and further including means providing pump power forachieving a population inversion of one of a pair of transitionscomprising a first transition between levels 3A and 1A,, and a secondtransition between levels 2A and 1A and means for stimulating emissionof transverse phonons of the frequency of said one transition comprisingmeans for enhancing repetitive reflection of phonons of the frequency ofsaid one transition along a predetermined axis of said crystal, andutilization means responsive to phonons at the frequency of said onetransition and propagated along said predetermined axis.

2. The emitter of claim 1 wherein said crystal and impurities areselected from the group consisting of OH impurities with any one of thealkali halides excepting KI, and CN impurities with any one of thealkali halides.

3. The emitter of claim 2 wherein said dipole is an hydroxyl ion.

4. The emitter of claim 3 wherein concentration of said ion in saidcrystal is in the range of l to 10 parts per million.

5 The emitter of claim 3 wherein said crystal is an alkali halideselected from the group consisting of KCI, NaCl, KBr, NaBr, LiCl, andLiBr.

6. The emitter of claim 5 wherein said alkali halide is potassiumchloride.

7. The emitter of claim 1 wherein said pumping means comprises a high Qcavity tuned to the frequency of the transition between 1A and 3A,levels, said crystal body being mounted within said cavity, and

means for energizing said cavity with a microwave electric field.

8. The emitter of claim 7 including means for varying the frequency ofemitted phonons comprising means for varying amplitude of said dc fieldto thereby change frequency of transition between said energy levels,and means for varying the tuned frequency of said cavity.

9. The emitter of claim 8 wherein said output means comprises a solidsonic conductor having an end portion mounted in abutment with an endportion of said crystal.

10. The phonon emitter of claim 1 wherein said means for enhancingreflection comprises flat parallel facing surface means on opposite endsof said crystal for repetitively reflecting phonons propagating alongsaid predetermined axis.

l l. A phonon amplifier comprising an alkali halide crystal havingelectric dipole impurities, means for subjecting the crystal to dc fieldsufficient to enable said impurity to assume energy said impurity havinga rate of decay from level 3A 1 *2A,, greater than its rate of decayfrom level 2A,

*1A,, and a rate of decay from level 1B 1A greater than its rate ofdecay from level 3A tunec l riiimping means for applying energy to saidcrystal at a frequency to raise said impurity to said 3A level, saidpumping means including means for achieving a population inversion ofone of a pair of transitions comprising a first transition betweenlevels 3A and 1B,, and a second transition between levels 2A and meansfor stimulating emission of phonons of the frequency of said onetransition comprising sonic coupling means for transmitting to saidcrystal an input vibration propagating along a predetermined axis at thefrequency of said one transition, and utilization means responsive tophonons at the frequency of said one transition and propagated alongsaid predetermined axis. 12. The phonon amplifier of claim 11 whereinsaid pumping means comprises a high Q cavity tuned to the frequency ofthe transition between [A and 3A levels, said crystal being mountedwithin said cavity, and means for energizing said cavity with amicrowave electric field. 13. The phonon amplifier of claim 12 includingmeans for varying the frequency of emitted phonons comprising means forvarying amplitude of said do field to thereby change frequency oftransition between said energy levels, and means for varying the tunedfrequency of said cavity. sonic 14. The phonon amplifier of claim 37wherein said coupling means comprises a solid sonic conductor having anend portion mounted in abutment with an end portion of said crystal.

2. The emitter of claim 1 wherein said crystal and impurities areselected from the group consisting of OH impurities with any one of thealkali halides excepting KI, and CN impurities with any one of thealkali halides.
 3. The emitter of claim 2 wherein said dipole is anhydroxyl ion.
 4. The emitter of claim 3 wherein concentration of saidion in said crystal is in the range of 1 to 10 parts per million.
 5. Theemitter of claim 3 wherein said crystal is an alkali halide selectedfrom the group consisting of KCl, NaCl, KBr, NaBr, LiCl, and LiBr. 6.The emitter of claim 5 wherein said alkali halide is potassium chloride.7. The emitter of claim 1 wherein said pumping means comprises a high Qcavity tuned to the frequency of the transition between 1A1 and 3A1levels, said crystal body being mounted within said cavity, and meansfor energizing said cavity with a microwave electric field.
 8. Theemitter of claim 7 including means for varying the frequency of emittedphonons comprising means for varying amplitude of said dc field tothereby change frequency of transition between said energy levels, andmeans for varying the tuned frequency of said cavity.
 9. The emitter ofclaim 8 wherein said output means comprises a solid sonic conductorhaving an end portion mounted in abutment with an end portion of saidcrystal.
 10. The phonon emitter of claim 1 wherein said means forenhancing reflection comprises flat parallel facing surface means onopposite ends of said crystal for repetitively reflecting phononspropagating along said predetermined axis.
 11. A phonon amplifiercomprising an alkali halide crystal having electric dipole impurities,means for subjecting the crystal to dc field sufficient to enable saidimpurity to assume energy levels 1A1, 2A1, 1B1, and 3A1, said impurityhaving a rate of decay from level 3A1 -> 2A1, greater than its rate ofdecay from level 2A1 -> 1A1, and a rate of decay from level 1B1 -> 1A1,greater than its rate of decay from level 3A1 -> 1B1, tuned pumpingmeans for applying energy to said crystal at a frequency to raise saidimpurity to said 3A1 level, said pumping means including means forachieving a population inversion of one of a pair of transitionscomprising a first transition between levels 3A1 and 1B1, and a secondtransition between levels 2A1, and means for stimulating emission ofphonons of the frequency of said one transition comprising soniccoupling means for transmitting to said crystal an input vibrationpropagating along a predetermined axis at the frequency of said onetransition, and utilization means responsive to phonons at the frequencyof said one transition and propagated along said predetermined axis. 12.The phonon amplifier of claim 11 wherein said pumping means comprises ahigh Q cavity tuned to the frequency of the transition between 1A1 and3A1 levels, said crystal being mounted within said cavity, and means forenergizing said cavity with a microwave electric field.
 13. The phononamplifier of claim 12 including means for varying the frequency ofemitted phonons comprising means for varying amplitude of said dc fieldto thereby change frequency of transition between said energy levels,and means for varying the tuned frequency of said cavity. sonic
 14. Thephonon amplifier of claim 37 wherein said coupling means comprises asolid sonic conductor having an end portion mounted in abutment with anend portion of said crystal.